In recent years, image forming apparatuses, which detect electromagnetic waves in frequency bands lower than the visible light region and the near-infrared region and image the intensity thereof on a two-dimensionally arranged pixel basis, have been developed. In such image forming apparatuses, imaging elements in which detection elements are integrated in a two-dimensional array are favorably used for performing imaging in one shot. This is because an imaging time is reduced considerably as compared with the imaging time in the method in which the individual detection elements are scanned. In that case, it is necessary that, typically, the detection elements be integrated together with control elements and the like on the same substrate.
PTL 1 discloses a technology to integrate Schottkey barrier diodes for millimeter waves on the same substrate as the substrate of heterostructure field-effect transistors. In this semiconductor device, a Schottkey barrier diode layer and a heterostructure field-effect transistor layer are stacked by epitaxial growth on a substrate in that order and an isolation layer serving as both an etching stopper layer and an insulating layer is provided between the layers. The Schottkey barrier diode is allowed to function as an element for detecting electromagnetic waves and, therefore, may be used for not only communications described in PTL 1 but also a millimeter wave imaging device by employing such a technology.
The configuration in which silicon (Si) is used as a relatively inexpensive semiconductor device has been studied. The mobility of Si is low as compared with the mobility of the III-V compound semiconductor because the microfabrication technology has been developed. Submicron scale microfabrication allows reduction of various time constants of structure causing delay in electromagnetic waves. In addition, Si has an advantage that it is possible to use a complementary metal oxide semiconductor (Complementary MOS, CMOS) as a control element.
NPL 1 discloses a technology to integrate the Schottkey barrier diode and the CMOS on the same substrate by using a standard CMOS process. In a disclosed method, the Schottkey barrier diode is formed by using the CMOS process, wherein in the CMOS process under 130 nm design rule, a technique to make a contact hole on an n-well structure and fill a metal into the contact hole is employed. The Schottkey barrier diode is integrated on a Si substrate together with a 280-GHz receiving antenna and a first-stage low noise amplifier (LNA) and serves as a high-sensitivity, low-noise detection element.
It is believed that such a configuration on the Si substrate is relatively inexpensive because production is performed by the standard CMOS process which has been completed as a process and, in addition, is suitable for an increase in the number of pixels in consideration of the diameters of wafers which have been already distributed.